The present invention relates to a false twist crimping machine of the type disclosed in U.S. Pat. Nos. 4,809,494, 3,962,829 and Re. No. 30,159.
False twist crimping machines serve to crimp endless thermoplastic filament yarns. Such filament yarns are initially spun as flat yarns, and in the false twist crimping machine, a plurality of yarns are processed in side by side parallel working stations. More particularly, each yarn is initially withdrawn from its supply package by a first feed system, then heated in a heater to about 220.degree. C., subsequently cooled, and then advanced through a false twist unit. A second feed system withdraws the yarn from the false twist unit.
The above described false twist method results in a permanent crimp being imparted to the yarn. Crimp values (in accordance with DIN [German Industrial Standards] 53840) amount to 40-50%. However, in many applications, attempts are made to reduce this final crimp behavior substantially. To this end, a secondary heating tube is used at each processing station. This heating tube is arranged between the second and a third feed system, and in the known false twist crimping machine, each tube externally heated. For heating, a heated vapor is preferably used, which achieves a uniformity of heating among the stations.
The effect of the heating tube is dependent on the method parameters of the heating zone. Decisive parameters are in particular the speed of the third feed system, the temperature of the heating tube, as well as its length. A certain deceleration of the yarn speed is generally provided by the third feed system, so as to reduce the yarn tension in the secondary heating tube. However, this deceleration, i.e. the speed difference between the second feed system and third feed system should not become too great, inasmuch as the yarn will slacken and result in yarn breakage. Furthermore, the threadline will become unstable, which leads to so-called voids, i.e. unevennesses of the yarn which later becomes visible in the fabric formed from the yarn.
It is accordingly an object of the present invention to improve and intensify the heating effect of the secondary heating tube of a yarn false twist crimping machine, by a novel design and construction of the heating tube.